Karadzic is a one-time ally of Slobodan Milosevic, on trial by the tribunal in The Hague, Netherlands on 66 counts of alleged genocide and other war crimes committed in Kosovo, Croatia and Bosnia. Each count carries a maximum sentence of life in prison.

With Milosevic in custody, the most prominent suspects still at large are Karadzic and his general, Ratko Mladic. The two Bosnian Serbs are wanted for genocide and crimes against humanity and would be potential witnesses at the trial of the former Yugoslav president.

Mladic is believed to be hiding in Belgrade, Serbia under the protection of the Yugoslav army, said Del Ponte's spokeswoman, Florence Hartmann.

Del Ponte's visit lasted only a few hours, underlining the single purpose of her trip _ to deliver a message to the Bosnian Serb leadership to start actively hunting suspects sought by her court. A lower-ranking prosecutor, Geoffrey Nice, is handling the day-to-day court duties in Milosevic's trial, which continued Friday.

Authorities in the Serb-controlled half of Bosnia have long refused to cooperate with the U.N. tribunal, ignoring its calls for the detention and deportation of suspects believed living there. While the Bosnian Serb parliament passed a law on cooperation with the tribunal in October, it still has not handed over any suspects.

Reacting to intense western pressure, Bosnian Serb leaders this week issued a decree effectively pledging that suspects who surrender within 30 days can expect their government to press for their freedom while awaiting trial by the tribunal. They also requested Yugoslavia to turn over any Bosnian Serb war crimes suspects hiding there.

But with more than six years gone without Bosnian Serb authorities detaining a single Serb suspect, Del Ponte said she remained "very unsatisfied and frustrated."

She said she wanted Karadzic to go on trial in October with Biljana Plavsic and Momcilo Krajisnik, both wartime allies of the former Bosnian Serb leader.

Prime Minister Mladen Ivanic denied knowledge of where Karadzic was.

Some 200,000 people were killed during the Bosnian war. A peace agreement divided the country into the Muslim-Croat federation and the Bosnian Serb republic, each with its own governments but linked on the federal level.